Process for the oxidation of cycloalkanes



United States Patent Int. Cl. C07f 5/043 C07c 35/08, 49/30 US. Cl. 260-462 7 Claims ABSTRACT OF THE DISCLOSURE Cycloalkanols and cycloalkanones are prepared by contacting a cycloalkane having 6 to 12 ring carbon atoms with a tri(organoperoxy)borane.

The present invention concerns the oxidation of cycloalkanes (alicyclic hydrocarbons), and more especially cyclohexane.

It is known to oxidise cyclohexane in the liquid phase, with oxygen or an oxygen-containing gas (such as air), into a mixture containing cyclohexanol and cyclohexanone. Cyclohexanone is valuable for the production of adipic acid, and hence polyamides; cyclohexanol can readily be dehydrogenated to cyclohexanone.

It is generally acepted that the first stage of the oxidation leads to cyclohexyl hydroperoxide, which is then converted by thermal decomposition into cyclohexanol and cyclohexanone, but if the operation is carried out without any particular precautions, the oxidation does not stop at this stage and more highly oxidised substances, more particularly undesirable acid substances, are formed as a result of secondary reactions, which reduce the yields of cyclohexanol and cyclohexanone.

In order to improve the yield of cyclohexanol and cyclohexanone relative to undersired by-products, the reaction can be carried out in the presence of certain oxygen-containing boron compounds. For example, it is Well known to oxidise cyclohexane with an oxygen-containing gas in the presence of a boric acid or of a boric anhydride capable of reacting with the cyclohexanol formed (see Belgian patent specification No. 635,117). However, this process involves either the elimination of water formed during esterification with the anhydride or the dehydration of the boron derivatives when acids are used. The only simple procedure for doing this consists in distilling the water-cyclohexene azeotrope. After cooling, the war ter is separated and the hydrocarbon is recycled. In order that this supply of cold cyclohexane may not interfere with the reaction conditions, it is necessary to pre-heat it during the recycling or in some other way supply heat to the system. In addition, this process has the disadvantages that it involves the use of solid boron derivatives, because the user encounters the problem of separating them and continuously introducing them into an apparatus in which the pressure is greater than atmospheric. Moreover, the recovery of these derivatives from their esters formed in the course of the reaction has been effected by a hydrolysis step which, despite the improvements describedin Belgian patent specification No. 635,- 301, involves many handling problems.

An attempt has been made to obviate the disadvantages inherent in the use of solid boron derivatives by carrying out the oxidation of hydrocarbons in the presence of C -C alkyl boric esters (see French patent specification No. 1,305,852), but the selectivity of the process, i.e. the preferential formation of cyclohexanol and of cyclohexanone in the case where the hydrocarbon is cy clohexane, is dependent upon the use of a considerable quantity of methyl borate, which is undesirable in industrial operation.

It is also known to prepare a solution of cyclohexyl hydroperoxide in cyclohexane by air oxidation of cyclohexane and stopping the oxidation when a relatively low proportion of hydroperoxide has been formed, and thereafter converting the hydroperoxide into cyclohexanol and cyclohexanone. This conversion may be effected for example, by chemical reduction either with hydrogen in the presence of catalysts (e.g. platinum or Raney nickel) or with salts of metals wherein the metal is in its lowest valance state, e.g. ferrous sulphate. It has also been suggested that the conversion of the cyclohexyl hydroperoxide be carried out in situ. Thus, according to British patent specification No. 777,087, the conversion is carried out in the presence of catalysts derived from cobalt or from chromium, but the quantities of cyclohexanol and cyclohexanone obtained show that it does not take place with satisfactory selectivity. It has also been proposed in British patent specification No. 892,723, to prepare borates of cycloalkanols from hydroperoxides, cycloalkanes, and boric acids or anhydrides. However, such a process is obviously attended by the abovementioned disadvantages, primarily the introduction, separation, and regeneration of solid boron derivatives.

It has now been found, and this forms the subject of the present invention, that it is possible to obtain excellent yields of cycloalkanol and cycloalkanone by reacting a saturated alicyclic hydrocarbon of formula RH (I), R being a saturated alicyclic hydrocarbon residue, with a tri(organoperoxy)borane of formula B(OOR') (II), in which R represents a monovalent hydrocarbon radical containing at least 4, normally 4 to 12 carbon atoms.

The process of the invention may be carried out by heating a solution of the tri(organoperoxy)borane (II) in the cycloalkane (I), optionally in the presence of a lower alkyl orthoborate of Formula IV given hereafter, at a temperature between and 180 'C. and preferably between and C. The lower alkanol formed from the lower alkyl orthoborate may, if desired, be eliminated as it is formed. Any cycloalkyl orthoborate formed may be treated by any known means to liberate the cycloalkanol. The reaction time will ordinarily be from /2 to 6 hours but the reaction is best continued until all the hydroperoxide is consumed or until substantially all the volatile lower alkanol has been removed.

The saturated alicyclic hydrocarbon of Formula (I) which may be oxidised by the process of the invention are cycloalkanes having from 6 to 12 ring carbon atoms, and are ordinarily unsubstituted cycloalkanes such as cyclohexane, cyclooctane and cyclododecane.

The tri(organoperoxy)boranes of formula B(OOR) include those wherein R represents the residue of a primary, secondary or tertiary alcohol containing at least 4 carbon atoms which may be aliphatic, alicyclic or arylaliphatic. More specifically, R may represent, for example, a butyl, pentyl, hexyl, octyl, dodecyl, cyclohexyl, methylcy-clohexyl, ethylcyclohexyl, cyclohexenyl, benzyl, phenylethyl, cumyl, p-nitrocumyl, tetrahydronaphthyl, methyltetrahydronaphthyl, decahydronaphthyl, indanyl, or pinanyl radical. The tri(organoperoxy)boranes may be obtained by the action of boron trichloride on hydroperoxides [Davis and Moodie, J. Chem. Soc. 2372 (1958)]. Another (more advantageous) method of preparation consists in reacting a hydroperoxide at a temperature below 90 C. with a lower alkyl orthoborate, under a pressure at which the alcohol liberated is removed by distillation as it is formed. This reaction may be generally represented by the equation:

number of moles of orthoborate number of moles of hydroperoxide is at least 5:1, it is advantageous to operate in a liquid solvent which is inert under the operating conditions. Examples of inert solvents are saturated aliphatic or alicyclic hydrocarbons and benzene hydrocarbons, which hydrocarbons may be unsubstituted or substituted by substituents such as halogen atoms, e.g. chlorine and fluorine. The solvent should desirably be chosen to give an efiicient elimination from the reaction mixture of the alcohol liberated by the reaction of the alkylorthoborate with the hydroperoxide and preferably solvents which favour the removal of the alcohol. Thus, for example, solvents forming a binary azeotrope with the alcohol are particularly suitable.

When a solvent is employed for the preparation of the tri(organoperoxy)boranes, stoichiometric quantities of hydroperoxide and alkyl orthoborate can be employed, but, as stated above, we prefer to operate with an excess of orthoborate, the precise molar ratio depending on the reactants. In general the molar ratio of orthoborate to hydroperoxide may be between 0.4 and 2. The proportion by weight of hydroperoxide relative to the solvent may vary widely, for example it may vary between 1% and preferably between 2% and 10%. Such solutions may be obtained by diluting relatively pure hydroperoxide.

The process of the invention is particularly applicable to tri(organoperoxy)boranes of the Formula (II) wherein R represents a saturated alicyclic radical, These tri(cycloalkylperoxy)boranes may be prepared by reacting a solution of the cycloalkyl hydroperoxide in the corresponding cycloalkane with an alkyl borate. The hydroperoxide solutions may be prepared by dissolving the pure hydroperoxide in the cycloalkane. The hydroperoxide should preferably have been purified, for example by converting it into its sodium salt and then treating it with carbon dioxide [Farkas et al. J. Amer. Chem. Soc. 72, 3333 (1950)]. The solutions may also be obtained directly and continuously without having first to isolate the hydroperoxide, by partially oxidising the cycloalkane in air, provided that the traces of acid formed in the course of this oxidation are removed. Not only do these acid by-products serve no purpose, but they may subsequently impair the stability of the peroxide derivatives. Besides the hydroperoxide, the

partial oxidation of the cycloalkane by air generally leads to small quantities of cycloalkanone and cycloalkanol which is converted into cycloalkyl borate during the preparation of the peroxyborane; these products do not interfere with the subsequent operations.

Of the tri(cycloalkylperoxy)boranes as defined above, tri(cyclohexylperoxy)borane may be employed with particular advantage. It may be prepared, for example, by treating in the manner hereinbefore stated a cyclohexyl hydroperoxide in solution in cyclohexane with a lower alkyl borate such as methyl orthoborate or ethyl orthoborate, the operation being carried out under atmospheric pressure. Ethyl orthoborate is preferably employed, because the ethanol liberated by the reaction may be easily eliminated by distillation of the ethanol-cyclohexane azeotrope.

The process of the invention may be carried out in a diluent which is inert under the operating conditions, but it is generally preferred to employ as diluent an excess of the cycloalkane which may thereafter be recovered or recycled. This excess is usually large; between 2% and 5% of tri(organoperoxy)borane calculated on the weight of the cycloalkane is generally suitable.

When the mixture has a boiling point at atmospheric pressure below the temperature chosen for the reaction, the reactants may be introduced into an apparatus under greater than atmospheric pressure, in order to maintain the mixture in the liquid phase, preferably at boiling point. This pressure, usually from 3 to 10 kg./cm. absolute, is generally supplied by a gas which is inert under the operating conditions, such as nitrogen.

The reaction of the tri(organoperoxy)borane with the cycloalkane may advantageously be carried out in the presence of a lower alkyl orthoborate of Formula (IV) given hereinbefore, which protects the cycloalkanol formed by transesterifying it as it is formed into the cycloalkanol orthoborate. The lower alkanol thus liberated is best removed from the reaction mixture. The proportion of orthoborate employed is not critical and quantities between 2 and 5 mol per mol of tri(organoperoxy)borane are generally suitable.

When the reaction has 'been carried out, the excess of orthoborate (IV) and the unoxidised cycloalkane (I) may be recovered or recycled. The residue, consisting essentially of cycloalkyl borate of Formula B (OR) (V), orthoborate B (OR) (VI) and cycloalkanone, lends itself preferably to the known methods of liberating a cycloalkanol from its borate, for example, by alcoholysis by means of a lower alkanol. Either the compound (V1) is eliminated before the residue is subjected to the alcoholysis, or the residue is reacted as it is and thereafter the alcohol ROH and the cycloalkane ROH are separated, for example by distillation. When the residue consisting mainly of cycloalkylborate and cycloalkanone is subjected to alcoholysis to liberate the cycloa kanol from its borate, the lower alkanol removed during the formation of the tri(organoperoxy)borane and during the reaction of the solution of tri(organoperoxy)borane with the the cycloalkane may be used.

A particular embodiment of the process of the invention consists in employing a tri(cycloalkylperoxy)borane obtained from the hydroperoxide derived from the cycloalkane to be oxidised, i.e. employing a hydrocarbon (I) and a tri(organoperoxy)borane (II) in which the radicals R and R are identical. Thus, if the diluent in the preparation of the tri(cycloalkylperoxy)borane is the cycloalkane to be reacted according to the process of the invention and if a suitable quantity of lower alkyl orthoborate is used, the solution of tri(cycloalkylperoxy)borane thus obtained may then be directly reacted with the cycloalkane. It may however be found desirable to dilute the tri(cycloalkylperoxy)borane solution with the cycloalkane, so as to bring the concentration of the former to the desired value.

When the reaction is complete, excess cycloalkane and alkyl borate may be removed from the final reaction mixture, and recycled for use in a subsequent operation.

It will be evident from the foregoing description that the process according to the invention is particularly useful for oxidising cyclohexane into cyclohexanol and cyclohexanone and the simplicity of its technical aspects is such that it meets the requirements for continuous industrial operation. For example, a solution of hydroperoxide in cyclohexane and ethyl borate may be injected into a first column heated at a temperature below C., and the reaction mixture may thereafter be introduced into a second column heated between and C, under the above-described pressure conditions. The cyclohexane and the ethyl borate may be separated from the residual mixture and recycled, respectively, into an apparatus for air oxidation, and into the first column. The heavy (non-volatile) products may then be introduced into a third column into which the ethanol-cyclohexane azeotrope is distilled from the first and second columns is simultaneously injected. The cyclohexanol and the cyclohexanone formed are then separated, and the ethyl borate can be recycled into the first column.

The following illustrative examples show how the invention can be put into practice:

Example 1 The apparatus employed consists of a 1 litre cylindrical vessel which may be externally heated, and which is provided with an efficient distillation column connected to a receiving system. The vessel and the column are made of stainless steel and the apparatus has seals for operation under greater than atmospheric pressure.

101.2 g. of a solution containing 4.5 g. of cyclohexyl hydroperoxide in cyclohexane, and 11.2 g. of ethyl orthoborate are introduced into the vessel. The apparatus is purged with dry nitrogen, whereafter the mixture is heated under atmospheric pressure until it boils; boiling starts at 80 C., and the ethanol is distilled as it is formed as a binary cyclohexane-ethanol azeotrope (B.P.=65 C.). The distillation is continued until the distillate contains pure cyclohexane.

The remaining 92 g. of solution are diluted with 130 cc. of cyclohexane, and are heated to 165 C. under pressure. The mixture boils under a pressure of 6.8 bars (6.9 kg./cm. and a liquid mixture is distilled which consists essentially of the binary ethanol-cyclohexane azeotrope.

The heating is stopped and the greater part of the cyclohexane is removed by expansion to atmospheric pressure. The residual cyclohexane and ethyl orthoborate are thereafter distilled.

After ethanolysis of the residue, 5.08 g, of cyclohexanol and 0.44 g. of cyclohexanone are isolated from the reaction product, i.e. represents 1.31 mol of cyclohexanol and 0.11 mol of cyclohexanone per mol of hydroperoxide introduced.

Example 2 The apparatus consists of a 3.6 litre autoclave provided with a stirring system and an efiicient distillation column connected to a receiving system, and is scalable for operation under pressure.

This is autoclave is charged with 1592 g. of a solution in cyclohexane of 66.7 g. of tri(cyclohexylperoxy)borane (4.19% by weight of the total solution charged), 82 g. of ethyl orthoborate, 12 g. of cyclohexyl orthoborate, 7.8 g. of cyclohexanone, and 8 g. of esters. The tri(cyclohexylperoxy)borane is obtained as a cyclohexane solution prepared from a technical solution in cyclohexane of cyclohexyl hydroperoxide and ethyl orthoborate by heating the reactants and simultaneously distilling the ethanol-cyclohexane azeotrope. The cyclohexyl hydroperoxide is obtained as a solution in cyclohexane prepared from the oxidation of cyclohexane by air, in the liquid phase, without catalyst, the oxidation being limited to about 4% conversion. Before being used, this solution was washed with Water and then with an aqueous sodium bicarbonate solution.

The solution is heated to 160 C. under pressure. The boiling is effected under a pressure of 6.5 bars (6.6 kg./cm. and 112.5 g. of a mixture of ethanol and cyclohexane are distilled for 30 minutes. The heating is then stopped and the greater part of the cyclohexane is removed by expansion to atmospheric pressure and the re mainder by distillation. 1150 g. are thus obtained of cyclohexane containing 6.32 g. of entrained cyclohexanone.

The residual product, which contains mainly cyclohexyl orthoborate and cyclohexanone, is alcoholysed by the addition of 400 cc. of methanol and heating at boiling point. 427 g. of a mixture of methyl orthoborate and methanol, containing 1.1 g. of entrained cyclohexanone, are simultaneously distilled.

The residual volatile products are distilled under 100 mm. Hg pressure. The residue consists of 105.5 g. of heavy products containing 80.55 g. of cyclohexanol and 12.85 g. of cyclohexanone, 1.23 mol of cyclohexanol and 0.09 mol of cyclohexanone per mol of hydroperoxide employed.

Example 3 An apparatus consisting of a cylindrical vessel (height: 2 metres) having a useful capacity of 3.3 litres, and surmounted by an efficient distillation column connected to a receiving system is employed. The cylinder and the column are made of stainless steel and the apparatus is scalable for operation under pressure. The cylinder has an external jacket through which a fluid maintained at C. is circulated.

16 kg. of the mixture of the technical tri(cyclohexylperoxy)borane and ethyl orthoborate solutions employed in the proceding example are injected into the base of the cylinder, at a rate of 3 litres per hour. The mixture is boiled under pressure of 7 bars (7.1 kg./cm. and, during the operation, 1350 g. of a mixture of cyclohexane and ethanol are distilled. The liquid reaction phase, which continuously overflows into a receiver, still contains 0.2% by weight of tri(cyclohexylperoxy)borane. 500 g. of this liquid phase are treated as in the preceding example, the alcoholysis being eflected by adding 150 ml. of methanol and heating. The various light distillates contain in all 2.7 g. of cyclohexanone.

The residue consists of 34.5 g. of heavy products containing 28 g. of cyclohexanol and 3 g. of cyclohexanone, 1.24 mol of cyclohexanol and 0.16 mol of cyclohexanone per mol of the hydroperoxide.

We claim:

1. Process for the production of a cycloalkanol and cycloalkanone which comprises contacting a cycloalkane having from 6 to 12 ring carbon atoms in a reaction zone at from 120 to C. with a preformed tri(organoperoxy)borane of formula:

where R is such that RH is the said cycloalkane.

2. Process according to claim 1, wherein the weight of tri(organoperoxy)borane is between 2% and 5% of the total weight of tri(organoperoxy)borane and cycloalkane.

3. Process according to claim 1, wherein the oxidation is effected at a pressure such that the reaction mixture boils within the stated temperature range, and the reaction is carried out at the boiling point.

4. Process according to claim 1, wherein the cycloalkane is an unsubstituted cycloalkane.

5. Process according to claim 4, wherein the cycloalkane is cyclohexane.

6. Process according to claim 1, wherein the cycloalkanol produced is converted, in said reaction zone into its borate ester as it is produced, by reaction with an alkyl orthoborate of formula B(OR") in which R is alkyl of 1 to 4 carbon atoms.

7. Process according to claim 6, wherein the cycloalkanol is regenerated from its borate ester by reaction with an alkanol of 1 to 4 carbon atoms.

References Cited FOREIGN PATENTS 1,361,232 4/ 1964 France.

892,723 3/ 1962 Great Britain. 1,008,314 10/1965 Great Britain.

OTHER REFERENCES Steinberg, Organoboron Chemistry, vol. I, p. 481 (1964).

LEON ZITVER, Primary Examiner M. M. JACOB, Assistant Examiner US. Cl. X.R. 

